Values in vector showing 0 when printed - c++

Question

I have been practicing with vectors, when I try to print the values with the length set, output is 0, but when I try to print the values without the length set, it shows correct values.
A question like this has been answered before but I didnt really get it.
Here's the code:-
#include <iostream>
#include <vector>
using namespace std;
int main()
{
int size,val;
cout << "Enter vector length " << endl;
cin >> size;
vector<int> vec(size);
for(int i = 0; i < size; i++){
cout << "Enter number for index " << i+1 << endl;
cin >> val;
vec.push_back(val);
}
for(int i = 0; i < size; i++){
cout << "Numeber at index " << i+1 << " is " << vec[i] << endl;
}
}
Output:-
Enter vector length
3
Enter number for index 1
1
Enter number for index 2
2
Enter number for index 3
3
Numeber at index 1 is 0
Numeber at index 2 is 0
Numeber at index 3 is 0
With value not set:-
#include <iostream>
#include <vector>
using namespace std;
int main()
{
int size,val;
cout << "Enter vector length " << endl;
cin >> size;
vector<int> vec;
for(int i = 0; i < size; i++){
cout << "Enter number for index " << i+1 << endl;
cin >> val;
vec.push_back(val);
}
for(int i = 0; i < size; i++){
cout << "Numeber at index " << i+1 << " is " << vec[i] << endl;
}
}
Output:-
Enter vector length
3
Enter number for index 1
1
Enter number for index 2
2
Enter number for index 3
3
Numeber at index 1 is 1
Numeber at index 2 is 2
Numeber at index 3 is 3
Help!!!

Answer 1

When you call push_back on a vector you are adding items to the end of it.
In the second example you push_back to it 3 times to the length of the vector is 3.
[] <-- vector is constructed
[ 1 ] <-- push_back(1)
[ 1, 2 ] <-- push_back(2)
[ 1, 2, 3 ] <-- push_back(3)
In the first example, when you set the length to be 3, those values are initialized to zero. Then you push_back 3 times so the final length of the vector is 6 and the first 3 values are still 0.
[ 0, 0, 0 ] <-- vector is constructed
[ 0, 0, 0, 1 ] <-- push_back(1)
[ 0, 0, 0, 1, 2 ] <-- push_back(2)
[ 0, 0, 0, 1, 2, 3 ] <-- push_back(3)
If you want to increase the CAPACITY of the vector but not the SIZE you want vector.reserve

Answer 2

There are two ways to do what you are trying to do.
One is the way you did it in your second trial.
The other is to modify your first trial's code:
Instead of vec.push_back(val);, try accessing the index directly:
vec.at(i) = val;
Both methods work fine.
The issue with your first trial was you resized a vector then appended values instead of doing what you thought it would do.
Basically, the vector was resized to size so all the elements from index 0 to size-1 were all set to 0, which is the default int value for vectors. After that, you appended new integers to the back of the vector. The reason your second trial worked is because the size of the vector started at 0, then you appended new integers to the back with your calls to push_back(int) in your for loop.
An important note: I would rephrase your output. Instead of saying "Number at index" << i + 1, change it to Number at element. The index numbering starts at 0. Element numbering starts at 1. As in: element 1 is located at index 0.